The present invention relates to a novel rice hull ash of ultra high fineness and other particular properties, as described fully hereinbelow, aqueous slurries of said ash and to their use as an admixture for hydraulic cement to enhance the properties of hydraulic cement compositions and the resultant set products made therewith.
Rice hull materials, such as the shell, hull or husk of the rice grain, are generally a discarded waste product from rice production. The rice hull materials are viewed as having little, if any, commercial value in their raw material form. It is known, however, that rice hull materials typically contain a substantial amount, typically about 16 to 20 percent, of silica (SiO.sub.2) and when burned yield an ash which is rich in amorphous silica. This ash, referred to herein and in the appended claims, as "rice hull ash" or "RHA", has been used as a pozzolanic additive in hydraulic cement compositions. The RHA is viewed as a filler material capable of replacing or extending the more expensive ingredients of the composition, such as portland cement and the like. (See U.S. Pat. No. 4,105,459 to P. K. Mehta.) The RHA material has also been proposed as a filler or extender in other applications, such as in elastomeric and plastimeric compositions (see U.S. Pat. No. 3,951,907 to P. K. Mehta). Its use in these general manners have value in countries where rice is a major crop and where the other components which are replaced or extended are expensive and/or not readily available.
The rice hull material is commonly converted into RHA by uncontrolled combustion methods in which the material is used as the fuel source. More recently, U.S. Pat. No. 3,959,007 disclosed a process in which a higher amount of energy is obtained when burning rice hull material under controlled combustion while still obtaining a RHA useful as a cement pozzolan or as an extender in other applications. The ash obtained from conventional modes of combustion may be further processed by dry grinding the ash to a powder form. The resultant ground material is a fluffy powder of low bulk density having a BET surface area of about 10 sq. m./g. or greater and, typically, a Blaine specific surface area of lower than 1 sq.m./gm. (The higher the Blaine value the smaller the particle.) Although the BET specific surface area of RHA preparations increases as the combustion temperature is reduced, and can be in excess of 200 sq.m./g., the Blaine specific surface areas of RHA's are usually much lower than this, typically less than 1 sq.m./g. This difference is because most of the specific surface measured by the BET technique is internal to the particle while the internal surfaces of a particle are not measured by the Blaine technique. The Blaine specific surface area measurements are greatly affected by particle size changes and are more indicative of particle size. Thus, the RHA material presently obtained and used is a somewhat coarse particle, having a low bulk density which makes the RHA hard to handle and deliver in desired amounts into a mix.
In the case where RHA is viewed predominantly as an extender-filler, the particle size of the RHA is not deemed to be critical as long as it is not so large as to disrupt the matrix to which it is added. In the case of its use as a pozzolan in cement compositions, it is known that small increases in the strength of the resultant hardened cement composition can be achieved by the use of smaller particle size RHA. However, this relationship tends to plateau as the particle size reaches a Blaine surface area of 1 to 1.5 sq. m/g. Therefore, there has been no incentive to attempt to develop a means to further reduce the RHA particle size and achieve a resultant product.
In summary, rice hull ash (RHA) is generally of high silica content (at least about 85% SiO.sub.2), but its usefulness as an additive in hydraulic cement mixes has heretofore been limited by the difficulty in obtaining it and handling it in a controlled, convenient and efficient manner. It has been demonstrated that the pozzolanic reactivity of RHA may be enhanced by burning the rice hulls at relatively low temperatures in specially-designed furnaces (U.S. Pat. No. 3,959,007). The strength of hardened hydraulic cements can be enhanced to small extents by using smaller particle RHA but this relationship plateaus. Because of this as well as the mechanical restrictions in dry grinding and the required need of specialized combustion techniques, there has been no desire to produce a RHA of very high Blaine value. Additional barriers to the need to form and use a high-Blaine RHA are the difficulty believed associated with handling the ultrafine dry powder, its presumed poor flow properties and with its dust hazards.